1. Field of the Invention
The present invention relates to a method and apparatus for polishing a workpiece, and more particularly to a method and apparatus for polishing a planar workpiece such as a semiconductor wafer to a flat mirror finish.
2. Description of the Related Art
Recent rapid progress in semiconductor device integration demands smaller and smaller wiring patterns or interconnections and also narrower spaces between interconnections which connect active areas. One of the processes available for forming such interconnection is photolithography. Though the photolithographic process can form interconnections that are at most 0.5 .mu.m wide, it requires that surfaces on which pattern images are to be focused by a stepper be as flat as possible because the depth of focus of the optical system is relatively small. Conventionally, as apparatuses for planarizing semiconductor wafers, there have been used a self-planarizing CVD apparatus, an etching apparatus or the like, however, these apparatuses fail to fully planarize semiconductor wafers. Recently, attempts have been made to use a polishing apparatus for planarizing semiconductor wafers to a flatter finish with more ease than those conventional planarizing apparatus.
Conventionally, a polishing apparatus has a turntable and a top ring which rotate at respective individual speeds. A polishing cloth is attached to the upper surface of the turntable. A semiconductor wafer to be polished is placed on the polishing cloth and clamped between the top ring and the turntable. An abrasive liquid containing abrasive grains is supplied onto the polishing cloth and retained on the polishing cloth. During operation, the top ring exerts a certain pressure on the turntable, and the surface of the semiconductor wafer held against the polishing cloth is therefore polished by a combination of chemical polishing and mechanical polishing to a flat mirror finish while the top ring and the turntable are rotated. This process is called Chemical Mechanical polishing.
After the semiconductor wafer is polished, it is detached from the top ring, and transferred to a next process such as a cleaning process.
While a workpiece such as a semiconductor wafer is being polished, it occasionally is broken into pieces which are scattered on the polishing cloth. Each time the workpiece being polished is broken, the polishing cloth has to be replaced with a new one because fragments of the workpiece on the polishing cloth would otherwise cause damage to the surface of another workpiece to be polished. Another problem is that a workpiece such as a semiconductor wafer which is being polished is sometimes disengaged from the top ring due to lack of secure attachment of the workpiece to the top ring. If the workpiece is a fragile object such as a semiconductor silicon wafer, then an outer circumferential edge of the workpiece may suffer same damage such as chipping upon collision with a wall surface that covers the turntable. When the damaged workpiece is polished again, it can easily be broken apart under small forces which are applied to the damaged area or thereabouts of the workpiece.
There have been made various efforts to prevent a workpiece from being broken and also from being disengaged from the top ring. For example, a cushioning member such as an elastic mat is interposed between the top ring and the workpiece to reduce cracking or chipping of the workpiece while the workpiece is being polished. A retainer ring is provided on the outer periphery of the top ring to retain the outer circumferential edge of the workpiece and to firmly hold the workpiece on the top ring for thereby preventing the workpiece from being disengaged from the top ring while the workpiece is being polished.
However, the above attempts fail to completely prevent the workpiece from being broken or being disengaged from the top ring while the work piece is being polished. Since the polishing process is continued even when the workpiece is broken or disengaged from the top ring, various resultant problems have not been eliminated.